1. Field of the Invention
The invention relates to a new polyisocyanate mixture containing uretdione diisocyanates and prepared from mixtures of 1,6-diisocyanatohexane (HDI) and 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), a process for the preparation of this polyisocyanate mixture and its use for the production of polyurethane surface coatings.
2. Description of the Prior Art
The preparation of polyisocyanates containing uretdione groups by partially dimerizing aliphatic diisocyanates in the presence of suitable catalysts is known. DE-OS (German Published Specification) 3,030,513 describes the dimerization of IPDI using tris-(dialkylamino)-phosphines as the catalyst. According to DE-OS 3,227,779, the uretdiones of 2-methyl-1,5-diisocyanatopentane and 2-ethyl-1,4-diisocyanatobutane can be prepared in the same manner. The process of DE-OS 3,437,635 describes the use of H-active organic co-catalysts, such as alcohols or amines, for the dimerization of other aliphatic diisocyanates, such as HDI. However, tris-(dimethylamino)-phosphine is known to oxidize very rapidly in the presence of atmospheric oxygen to form the carcinogenic hexamethylphosphoric acid triamide which prohibits the commercial use of this catalyst.
Lewis acids, such as boron trifluoride or antimony pentafluoride, are proposed as dimerization catalysts for aliphatic diisocyanates in DE-OS 1,670,720 and 3,420,114. These processes have not become established, inter alia, because of the highly corrosive action of these catalysts.
According to DE-OS 3,739,549, dialkylamino-substituted pyridines, such as p-dimethylaminopyridine (DMAP), are suitable catalysts for the preparation of pure uretdiones from aliphatic diisocyanates. However, according to our own experiments, the products obtained from HDI and IPDI by this process display a marked intrinsic coloration, which renders them unsuitable for use as surface coating polyisocyanates.
The method known the longest for the dimerization of aliphatic diisocyanates comprises their modification
with the aid of tertiary phosphines, preferably with trialkylphosphines. This is the subject matter of DE-OS 1,670,720 and 1,934,763, and of U.S. Pat. No. 4,614,785. The products obtained in this manner always contain, in addition to uretdione groups, a greater or lesser content of isocyanurate groups, and are relatively dark in color. However, further experiments have constantly been undertaken to improve the process.
Low-viscosity polyisocyanates which contain uretdione groups and have a low color number can be prepared from HDI by the urethanization of a portion of the isocyanate groups with suitable alcohols before or during the oligomerization reaction and treatment of the resin obtained after thin film distillation with organic peroxides, as described in DE-OS 3900053. These products are particularly suitable for use in light-fast one- or two-component polyurethane surface coatings, but cannot meet all the necessary requirements.
Surface coating polyisocyanates based on HDI, in particular diisocyanates also containing uretdione groups and based on HDI, are inferior to the corresponding polyisocyanates based on IPDI with regard to the resistance of the coatings obtained from the polyisocyanates to weathering. The diisocyanates which contain uretdione groups and are based on HDI are moreover unsuitable as the starting material for the preparation of powder coatings, for example, because of their low viscosity. However, diisocyanates which contain uretdione groups and are based on IPDI are suitable for this application as demonstrated in, for example, EPA 0,045,998. However, the disadvantage of these uretdione diisocyanates is their poor production rates. The dimerization of IPDI with the trialkylphosphine catalysts preferably employed for the dimerization of aliphatic diisocyanates requires relatively high catalyst concentrations (1 to 3%) and very long reaction times (8 days to 8 weeks) as disclosed in DE-OS 1,934,763.
An object of the present invention is to provide new polyisocyanates containing uretdione groups which are predominantly based on dimerized IPDI and combine the good surface coating properties known of surface coating polyisocyanates based on IPDI and at the same time can be prepared in high space-time yields.
This object may be achieved by the process according to the present invention, described below in more detail, for the preparation of the new polyisocyanate mixtures. This process is based on the surprising observation that polyisocyanates which contain uretdione groups and in which a very high proportion of the cycloaliphatic diisocyanate is incorporated can be prepared very easily from mixtures of HDI and IPDI having a comparatively low content of HDI by modification with trialkylphosphines.
Even the preparation of highly viscous to solid products under conditions which are appropriate in practice, i.e., low catalyst concentrations and relatively short reaction times, is made possible in this manner. Some of the above-mentioned publications, for example, U.S. Pat. No. 4,614,785 and DE-OS 3,420,114, 3,739,549 and 3,900,053, disclose long lists of suitable (cyclo)aliphatic diisocyanates and indicate that mixtures of these isocyanates can also be employed for the dimerization. However, it would not be possible for the skilled artisan to ascertain from these teachings that by admixing HDI to IPDI the reaction times for the preparation of uretdiones can be shortened drastically, even at a very high content of IPDI in the starting mixture, and that a large amount of IPDI can be incorporated into the polyisocyanate as demonstrated in the examples below. Based on the prior art these results are surprising since on the basis of the large difference in the reactivity of HDI and IPDI, it would be expected that the pure HDI uretdione and not an oligomer having a high IPDI content would preferentially be formed from mixtures of the two diisocyanates.